Since development of sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE), PAGE has become an indispensable means in the biochemical analyses of biosamples such as proteins and nucleic acids. As a means for detecting proteins on a polyacrylamide gel after electrophoresis, for example, the CBB staining, silver staining, fluorescent staining, and the like can be used. In comprehensive researches for analyzing entire features of proteins (proteome) in a biological tissue or cell to facilitate developments of pharmaceuticals, i.e., proteomics, a particularly high sensitive means is desired, and among the aforementioned means, the silver staining has been widely used in this field as a method that can provide the highest detection sensitivity without need of any special detection apparatus. When silver staining is performed by using a commercially available silver staining kit or the like currently provided, detection at a protein amount of several hundreds picograms or more is achievable.
However, even when a conventional silver staining method is used, 108 to 109 or more cells are required to detect a protein of which expression amount per cell is low. If recovery efficiency of the protein is taken into consideration, much more numbers of cells are required, and thus a problem arises that practical analysis is impossible. Under the circumstances, it is strongly desired to develop a method which is capable of clearly visualizing a protein on a gel even when PAGE is performed by using a small amount of the protein, for example, about several tens picograms.